Thermoelectric devices such as power generators, heat pumps, coolers and thermal sensors have advantages over traditional energy converting systems in several aspects: high reliability, portable weight, no maintenance required, and environmentally friendly. Thermoelectric devices, which can directly convert heat into electricity, could play an important role in the future of energy conversion, management, and utilization. However, the low efficiency of present energy conversion thermoelectric devices, limits the ability of those devices to completely or even partially replace the equipment in traditional energy converting systems.
The efficiency of thermoelectric materials is related to the dimensionless figure of merit ZT, where ZT=(σS2/κ)T and σ is the electrical conductivity, S the thermopower or absolute Seebeck coefficient, T is the temperature, and κ is the thermal conductivity. Currently, PbTe and Si/Ge alloys are the basic thermoelectric materials used for power generation and, once doped appropriately, can possess a maximum ZT of approximately 0.8 at 600 K and 1 at 1200 K, respectively.
Typically, there are two ways to improve the ZT of thermoelectric materials: one is to enhance the power factor (σS2) and the other is to lower the lattice thermal conductivity. Approaches to increase the power factor include introducing a resonance level in the valence band, e.g., in Tl—PbTe (see, J. P. Heremans et al., Enhancement of thermoelectric efficiency in PbTe by distortion of the electronic density of states. Science 321, 554-557 (2008) and S. Ahmad, K. Hoang, S. D. Mahanti, Ab Initio study of deep defect states in narrow band-gap semiconductors: group III impurities in PbTe. Phys. Rev. Lett. 96, 56403(1-4) (2006)) or by synergistic nanostructuring (see, J. R. Sootsman et al., Large enhancement in the power factor of bulk PbTe at high temperature by synergistic nanostructuring. Angew. Chem. Int. Ed. 47, 8618-8622 (2008)). Nanoscale inclusions in bulk materials can dramatically suppress the lattice thermal conductivity by scattering the longer wavelength heat-carrying phonons to achieve high ZT. Nanostructured bulk materials such as AgPbmSbTem+2 (see, K. F. Hsu, et al., Cubic AgPbmSbTe2+m: bulk thermoelectric materials with high figure of merit. Science 303, 818-821 (2004), E. Quarez, et al. Nanostructuring, compositional fluctuations, and atomic ordering in the thermoelectric materials AgPbmSbTe2+m. The myth of solid solutions. J. Am. Chem. Soc. 127, 9177-9190 (2005) and M. Zhou, J.-F. Li, T. Kita, Nanostructured AgPbmSbTe2+m system bulk materials with enhanced thermoelectric performance. J. Am. Chem. Soc. 130, 4527-4532 (2008)), AgPbmSnnSbTe2+m+n (see, J. Androulakis et al., Nanostructuring and high thermoelectric efficiency in p-type Ag(Pb1-ySny)mSbTe2+m. Adv. Mater. 18, 1170-1173 (2006)), NaPbmSbTe2+m (see, P. F. P. Poudeu et al., High thermoelectric figure of merit and nanostructuring in bulk p-type Na1-xPbmSbyTe2+m. Angew. Chem. Int. Ed. 45, 3835-3839 (2006)), PbTe—PbS (see, J. Androulakis et al., Spinoidal decomposition and nucleation and growth as a means to bulk nanostructured thermoelectric: enhanced performance in Pb1-xSnxTe—PbS. J. Am. Chem. Soc. 129, 9780-9788 (2007)) and BiSbTe (see, B. Poudel, et al., High-thermoelectric performance of nanostructured bismuth antimony telluride bulk alloys. Science 320, 634-638 (2008)) are examples of this approach. In all of these cases, however, the power factor also takes a hit because the nanostructuring simultaneously increases carrier scattering which adversely affects the carrier mobilities.